The present invention relates to a silicon single crystal wafer having few crystal defects, in particular, no OSF ring, and an N-region over its entire plane, and a method for producing the same.
In recent years, with the use of finer semiconductor devices required for higher integration level of semiconductor devices, there has been desired increasingly higher quality of silicon single crystals produced by the Czochralski method (abbreviated as CZ method hereinafter) and used for substrates of the devices. In particular, such crystals have defects introduced during the crystal growth, which are called grown-in defects such as FPD, LSTD and COP, and degrade the oxide dielectric breakdown voltage and other device characteristics, and it is considered important to reduce the density and the size of the defects.
For the reference of the explanation of those defects, there will be given first general knowledge of factors determining densities of defects introduced into silicon single crystals, a void type point defect called vacancy (occasionally abbreviated as V hereinafter), and an interstitial type silicon point defect called interstitial silicon (occasionally abbreviated as I hereinafter).
In a silicon single crystal, a V-region means a region containing many vacancies, i.e., depressions, pits and the like generated due to missing of silicon atoms, and an I-region means a region containing many dislocations and aggregations of excessive silicon atoms generated due to the presence of excessive amount of silicon atoms. Between the V-region and the I-region, there should be a neutral (occasionally abbreviated as N hereinafter) region with no (or little) shortage or no (or little) surplus of the atoms. It has become clear that the aforementioned grown-in defects (FPD, LSTD, COP etc.) should be generated strictly only with supersaturated V or I, and they would not be present as defects even though there is little unevenness of atoms so long as V or I is not saturated.
The densities of these two kinds of point defects are determined by the relationship between the crystal pulling rate (growing rate), and the temperature gradient G in the vicinity of the solid-liquid interface in the crystal in the CZ method. It has been confirmed that defects distributed in a ring shape called OSFs (Oxidation Induced Stacking Fault, the ring may be referred to as xe2x80x9cOSF ringxe2x80x9d hereinafter) are present around the boundary between the V-region and the I-region in a cross section perpendicular to the crystal growing direction, and it is considered important to reduce the density and the size of these defects as defects generated during the crystal growth and degrading the oxide dielectric breakdown voltage and other device characteristics.
Those defects generated during the crystal growth are categorized as follows. When the growth rate is relatively high, i.e., around 0.6 mm/min or higher, grown-in defects considered to be originated from voids, i.e., aggregations of void-type point defects, such as FPD, LSTD and COP are distributed over the entire plane of the crystal along the radial direction at a high density, and a region containing such defects is called V-rich region (see FIG. 5a). When the growth rate is 0.6 mm/min or lower, the aforementioned OSF ring is initially generated at the circumferential part of the crystal with the decrease of the growth rate, and L/Ds (large dislocations, also called interstitial dislocation loops, which include LSEPD, LFPD and the like), which are considered to be originated from dislocation loops, are present outside the ring at a low density, and a region containing such defects is called I-rich region (see FIG. 5b). When the growth rate is further lowered to around 0.4 mm/min, the OSF ring shrinks toward the center of wafer and disappears, and thus the entire plane becomes the I-rich region (FIG. 5c).
Recently, there has been discovered presence of a region called N-region containing neither the void-originated grown-in defects such as FPD, LSTD and COP, nor the dislocation loop-originated LSEPD and LFPD, which region is present between the V-rich region and the I-rich region, and outside the OSF ring (see Japanese Patent Laid-open Publication [Kokai] No. 8-330316). It was reported that this region existed outside the OSF ring, and showed substantially no oxygen precipitation observed when subjected to a heat treatment for oxygen precipitation and examined by X-ray analysis or the like as for the precipitation contrast, as well as this region was present at rather I-rich side where the defects were not so rich as to form LSEPD and LFPD (see FIG. 4a). Further, it was suggested that the N-region, which could be obtained only for an extremely small portion of a wafer obtained by a conventional CZ pulling apparatus, could be enlarged over the entire plane of wafer by improving temperature distribution in a furnace of the pulling apparatus and controlling the pulling rate, that is, by maintaining a F/G value (a ratio represented as F/G where F is a single crystal pulling rate [mm/min] and G represents an average temperature gradient along the crystal growing direction in the temperature range of from the melting point of silicon and 1300xc2x0 C. [xc2x0C/mm]) within a range of from 0.20 to 0.22 mm2/xc2x0Cxc2x7min for the entire plane of the wafer and the entire length of the crystal (see FIG. 4b).
However, if it is attempted to produce a crystal with such a region containing extremely few defects and enlarged over the entire crystal, this region is limited to the N-region on the I-rich region side. Therefore, controllable ranges of production conditions are very narrow. However, such precise control is difficult in apparatuses for industrial production, even though it may be possible in experimental apparatuses. Thus, such production suffers from a problem concerning productivity and is not practical.
Furthermore, it was found that the defect distribution chart disclosed in the aforementioned application and the data used for it were considerably different from the data obtained by the inventors of the present invention through experiments and researches and the defect distribution chart prepared based on the data (see FIG. 1).
The present invention was accomplished in view of the aforementioned problem, and its object is to obtain a silicon single crystal wafer of an extremely low defect density, which has the N-region for the entire plane of the crystal and neither of V-rich region, I-rich region and OSF ring, by the CZ method with easily controllable production conditions because of wide controllable ranges thereof, while maintaining high productivity.
The present invention was accomplished in order to achieve the aforementioned object, and provides a method for producing a silicon single crystal, wherein, when a silicon single crystal is grown by the Czochralski method, the crystal is pulled with such conditions as present in a region defined by a boundary between a V-rich region and an N-region and a boundary between an N-region and an I-rich region in a defect distribution chart showing defect distribution which is plotted with D [mm] as abscissa and F/G [mm2/xc2x0Cxc2x7min] as ordinate, wherein D represents a distance between center of the crystal and periphery of the crystal, F [mm/min] represents a pulling rate and G [xc2x0C/mm] represents an average temperature gradient along the crystal pulling axis direction in the temperature range of from the melting point of silicon to 1400xc2x0 C., and time required for crystal temperature to pass through the temperature region of from 900xc2x0 C. to 600xc2x0 C. is controlled to be 700 minutes or shorter.
If a crystal is pulled while the pulling rate F of the crystal and the average temperature gradient G along the crystal pulling axis direction in the temperature range of from the melting point of silicon to 1400xc2x0 C. are controlled so that the conditions should be present in a region defined by a boundary between a V-rich region and an N-region and a boundary between an N-region and an I-rich region in the defect distribution chart shown in FIG. 1, which was obtained through analysis of the results of experiments and researches, as described above, OSFs would be present in the plane, as will be generated in a ring shape during a thermal oxidation treatment, and thus a grown single crystal having the N-regions for all of the portions inside and outside the OSF ring would be obtained (see FIG. 3b). However, by further controlling the thermal history of the crystal so that the time required for crystal temperature to pass through the temperature region of from 900xc2x0 C. to 600xc2x0 C. should be 700 minutes or shorter, generation and growth of OSF nuclei can be inhibited, and therefore the OSF ring is not generated when the wafer is subjected to OSF thermal oxidation treatment, even though nuclei for the OSF ring are latently present therein. Thus, there can be obtained a wafer of extremely low defect density having N-region over the entire plane and neither of V-rich region, I-rich region and the harmful OSF ring (see FIG. 3c).
In this case, when the silicon single crystal is grown by the Czochralski method, it is desirable that oxygen concentration in the growing crystal is controlled to be 24.0 ppma or less.
By pulling the crystal while the oxygen concentration in the growing crystal is controlled to be 24.0 ppma (value according to ASTM ""79) or less as described above, generation and growth of OSF nuclei can surely be suppressed. Thus, there can be obtained a silicon single crystal wafer with extremely few crystal defects containing no OSFs, which are generated in a ring shape during the thermal oxidation treatment, even though there exist latent nuclei for the OSF ring.
The present invention also provides a silicon single crystal wafer produced from a silicon single crystal produced by the production method of the present invention. The silicon wafer obtained by the present invention is a silicon single crystal wafer having N-region for the entire plane, extremely few crystal defects and no OSF, which is generated in a ring shape during the OSF thermal oxidation treatment, while there exist latent nuclei for the OSF ring.
The present invention also provides a silicon single crystal wafer grown by the Czochralski method, which is a silicon single crystal wafer having N-region for its entire plane, and does not generate OSFs by a single-step thermal oxidation treatment, but generates OSFs by a two-step thermal oxidation treatment.
That is, the wafer of the present invention is a silicon single crystal wafer having N-region for the entire plane and extremely few crystal defects, in which latent nuclei of OSFs originally exist, and OSFs are not generated by a usual single-step thermal oxidation treatment, but the latent nuclei of OSFs are grown by a two-step thermal oxidation treatment to generate OSFs. These OSFs generated only by the two-step thermal oxidation treatment are not the defects that degrade the oxide dielectric breakdown voltage and other device characteristics in actual processes.
The present invention further provides a silicon single crystal wafer, which contains crystal portions in which oxygen is not precipitated among crystal portions in which oxygen is precipitated by an oxygen precipitation heat treatment.
Because the wafer of the present invention contains latent nuclei of OSFs, oxygen is not precipitated by the oxygen precipitation heat treatment in portions where the latent nuclei for the OSF ring are present, but oxygen is precipitated in the vicinity of such portions.
As explained above, according to the present invention, the controllable ranges of single crystal growth conditions become larger by selecting a suitable F/G value. Further, by using control of the thermal history in the low temperature region or control for obtaining low oxygen concentration in combination, there can be produced a silicon single crystal wafer having N-region for the entire plane and no crystal defects, wherein the OSF ring is not generated by a thermal oxidation treatment in spite of the presence of OSF latent nuclei and there are neither of grown-in defects and L/Ds in the plane.